The present invention relates to a process for preparing rare earth activated alkaline earth metal fluorohalide stimulable phosphor particles and rare earth activated alkaline earth metal fluorohalide stimulable phosphor particles, and in particular to a process for preparing rare earth activated alkaline earth metal fluorohalide stimulable phosphor particles without causing deterioration of performance due to moisture absorption and rare earth activated alkaline earth metal fluorohalide stimulable phosphor particles.
Radiographic images such as X-ray images are frequently employed for use in medical diagnosis. To obtain such X-ray images, radiography is employed, in which X-rays transmitted through an object are irradiated onto a phosphor layer (so-called fluorescent screen), thereby producing visible light, which exposes silver salt photographic film and the thus exposed film is developed in such a manner similar to that conducted in conventional photography. Recently, there has been introduced a technique of reading images directly from the phosphor layer, without using the silver salt photographic film. However, this phosphor layer exhibits relatively high hygroscopicity, thereby easily leading to lowering in sensitivity and therefore, a technique for enhancing moisture resistance has been strongly sought.
As such a technique, there is known a method, in which radiation transmitted through an object is allowed to be absorbed by a phosphor, followed by exciting the phosphor with light or thermal energy to release radiation energy stored therein as fluorescent light emission, and the emitted fluorescent light is detected to form images. Exemplarily, a radiation image conversion method using stimulable phosphors is known, as described in U.S. Pat. No. 3,859,527 and JP-A No. 55-12144 (hereinafter, the term, JP-A refers to an unexamined and published Japanese Patent Application).
In this method, a radiation image conversion panel containing a stimulable phosphor is employed. Thus, a stimulable phosphor layer of the radiation image conversion panel is exposed to radiation transmitted through an object to store radiation energies corresponding to respective portions of the object, followed by sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as xe2x80x9cstimulating raysxe2x80x9d) to release the radiation energy stored in the phosphor as light emission (stimulated emission), photo-electrically detecting the emitted light to obtain electric signals, and reproducing the radiation image of the object as a visible image from the electrical signals on a recording material such as photographic film or a CRT.
The foregoing radiation image recording and reproducing method has an advantage in that radiation images having abundant information content can be at a low exposure dose relative to conventional radiography using the combination of a conventional radiographic film and intensifying screen.
Stimulable phosphors are phosphor material that, after having been exposed to radiation rays, causes stimulated emission by exposing to stimulating rays. Phosphors capable of causing stimulated emission at a wavelength of 400 to 900 nm with a stimulating ray of 400 to 900 nm are generally applied to practical use.
Examples of the stimulable phosphor used in the radiation image conversion panel include,
(1) a rare earth activated alkaline earth metal fluorohalide phosphor represented by the formula of (Ba1-x, M2+x)FX:yA, as described in JP-A No. 55-12145, in which M2+ is at least one of Mg, Ca, Sr, Zn and Cd; X is at least one of Cl, Br and I; A is at least one of Eu, Tb, Ce, Tm, Dy, Pr, Ho, Nd, Yb, and Er; x and y are numbers meeting the conditions of 0xe2x89xa6xxe2x89xa60.6 and 0xe2x89xa6yxe2x89xa60.2; and the phosphor may contain the following additives:
Xxe2x80x2, BeXxe2x80x3 and M3X3xe2x80x2xe2x80x3, as described in JP-A No. 56-74175 (in which Xxe2x80x2, Xxe2x80x3 and Xxe2x80x2xe2x80x3 are respectively at least a halogen atom selected from the group of Cl, Br and I; and M3 is a trivalent metal);
a metal oxide described in JP-A No. 55-160078, such as BeO, BgO, CaO, SrO, BaO, ZnO, Al2O3, Y2O3, La2O3, In2O3, SiO2, TiO2, ZrO2, GeO2, SnO2, Nb2O5 or ThO2;
Zr and Sc described in JP-A No. 56-116777;
B described in JP-A No. 57-23673;
As and Si described in JP-A No. 57-23675;
Mxc2x7L (in which M is an alkali metal selected from the group of Li, Na, K, Rb and Cs; L is a trivalent metal Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Al, Ga, In, and Tl) described in JP-A 58-206678;
calcined tetrafluoroboric acid compound described in JP-A No. 59-27980;
calcined, univalent or divalent metal salt of hexafluorosilic acid, hexafluorotitanic acid or hexafluorozirconic acid described in JP-A No. 59-27289;
NaXxe2x80x2 described in JP-A No. 59-56479 (in which Xxe2x80x2 is at least one of Cl, Br and I);
a transition metal such as V, Cr, Mn, Fe, Co or Ni, as described in JP-A No. 59-56479;
M1Xxe2x80x2, Mxe2x80x22Xxe2x80x3, M3Xxe2x80x2xe2x80x3 and A, as described in JP-A No. 59-75200 (in which M1 is an alkali metal selected from the group of Li, Na, K, Rb and Cs; Mxe2x80x22 is a divalent metal selected from the group of Be and Mg; M3 is a trivalent metal selected from the group Al, Ga, In and Tl; A is a metal oxide; Xxe2x80x2, Xxe2x80x3 and Xxe2x80x2xe2x80x3 are respectively a halogen atom selected from the group of F, Cl, Br and I);M1Xxe2x80x2 described in JP-A No. 60-101173 (in which M1 is an alkali metal selected from the group of Rb and Cs; and Xxe2x80x2 is a halogen atom selected from the group of F, Cl, Br and I);
M2xe2x80x2Xxe2x80x22.M2xe2x80x2Xxe2x80x32 (in which M2xe2x80x2 is at least an alkaline earth metal selected from the group Ba, Sr and Ca; Xxe2x80x2 and Xxe2x80x3 are respectively a halogen atom selected from the group of Cl, Br and I, and Xxe2x80x2xe2x89xa0Xxe2x80x3); and
LnXxe2x80x33 described in JP-A No. 61-264084 (in which Ln is a rare earth selected from the group of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu; Xxe2x80x3 is a halogen atom selected from the group of F, Cl, Br and I);
(2) a divalent europium activated alkaline earth metal halide phosphor described in JP-A No. 60-84381, represented by the formula of M2X2.aM2xe2x80x22:xEu2+ (in which M2 is an alkaline earth metal selected from the group of Ba, Sr and Ca; X and Xxe2x80x2 is a halogen atom selected from the group of Cl, Br and I and Xxe2x89xa0Xxe2x80x2; a and x are respectively numbers meeting the requirements of 0xe2x89xa6axe2x89xa60.1 and 0 less than xxe2x89xa60.2);
the phosphor may contain the following additives;
M1Xxe2x80x3 described in JP-A No. 60-166379 (in which M1 is an alkali metal selected from the group of Rb, and Cs; Xxe2x80x3 is a halogen atom selected from the group of F, Cl, Br and I;
KXxe2x80x3, MgX2xe2x80x2xe2x80x3 and M3X3xe2x80x3xe2x80x3 described in JP-A No. 221483 (in which M3 is a trivalent metal selected from the group of Sc, Y, La, Gd and Lu; Xxe2x80x3, Xxe2x80x2xe2x80x3 and Xxe2x80x3xe2x80x3 are respectively a halogen atom selected from the group of F, Cl Br and I;
B described in JP-A No. 60-228592;
an oxide such as SiO2 or P2O5 described in JP-A No. 60-228593;
LiXxe2x80x3 and NaXxe2x80x3 (in which Xxe2x80x3 is a halogen atom selected from the group of F, Cl, Br and I;
SiO2 described in JP-A No. 61-120883;
SnX2xe2x80x2 described in JP-A 61-120885 (in which Xxe2x80x3 is a halogen atom selected from the group of F, Cl, Br and I;
CsXxe2x80x3 and SnX2xe2x80x2xe2x80x3 described in JP-A No. 61-235486 (in which Xxe2x80x3 and Xxe2x80x2xe2x80x3 are respectively a halogen atom selected from the group of F, Cl, Br and I;
CsXxe2x80x3 and Ln3+ described in JP-A 61-235487 (in which Xxe2x80x3 is a halogen atom selected from the group of F, Cl, Br and I; Ln is a rare earth element selected from the group of Sc, Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu;
(3) a rare earth element activated rare earth oxyhalide phosphor represented by the formula of LnOX:xA, as described in JP-A No. 55-12144 (in which Ln is at least one of La, Y, Gd and Lu; A is at least one of Ce and Tb; and x is a number meeting the following condition, 0x less than 0.1);
(4) a cerium activated trivalent metal oxyhalide phosphor represented by the formula of M(II)OX:xCe, as described in JP-A No. 58-69281 (in which M(II) is an oxidized metal selected from the group of Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb and Bi; X is a halogen atom selected from the group of Cl, Br and I; x is a number meeting the following condition, 0 less than x less than 0.1;
(5) a bismuth activated alkali metal halide phosphor represented by the formula of M(I)X:xBi, as described in JP-A No.62-25189 (in which M(I) is an alkali metal selected from the group of Rb and Cs; X is a halogen atom selected from the group of Cl, Br and I; x is a number meeting the following condition, 0 less than x less than 0.2;
(6) a divalent europium activated alkaline earth metal halophosphate phosphor represented by the formula of M(II)5(PO4)3X:xEu2+, as described in JP-A No. 60-141783 (in which M(II) is an alkaline earth metal selected from the group of Ca, Sr and Ba; X is a halogen atom selected from the group of F, Cl, Br and I; x is a number meeting the following condition, 0 less than xxe2x89xa60.2);
(7) a divalent europium activated alkaline earth metal haloborate phosphor represented by the formula of M(II)2BO3X:xEu2+, as described in JP-A No. 60 157099 (in which M(II) is an alkaline earth metal selected from the group of Ca, Sr and Ba; X is a halogen atom selected from the group of Cl, Br and I; x is a number meeting the following condition, 0 less than xxe2x89xa60.2);
(8) a divalent europium activated alkaline earth metal halophosphate phosphor represented by the formula of M(II)2PO4X:xEu2+, as described in JP-A No. 60-157100 (in which M(II) is an alkaline earth metal selected from the group of Ca, Sr and Ba; X is a halogen atom selected from the group of Cl, Br and I; x is a number meeting the following condition, 0 less than xxe2x89xa60.2);
(9) a divalent europium activated alkaline earth metal hydrogenated halide phosphor represented by the formula of M(II)HX:xEu2+, as described in JP-A No. 60-217354 (in which M(II) is an alkaline earth metal selected from the group of Ca, Sr and Ba; X is a halogen atom selected from the group of Cl, Br and I; x is a number meeting the following condition, 0 less than xxe2x89xa60.2);
(10) a cerium activated rare earth complex halide phosphor represented by the formula of LnX3.aLnxe2x80x2X3xe2x80x2:xCe3+, as described in JP-A No. 61-21173 (in which Ln and Lnxe2x80x2 are individually a rare earth element selected from the group of Y, La, Gd and Lu; X and Xxe2x80x2 are respectively a halogen atom selected from the group of F, Cl, Br and I and Xxe2x89xa0Xxe2x80x2; a and x are respectively numbers meeting the following conditions, 0.1 less than axe2x89xa610.0 and 0 less than xxe2x89xa60.2;
(11) a cerium activated rare earth complex halide phosphor represented by the formula of LnX3.aM(I)Xxe2x80x2:xCe3+, as described in JP-A 61-21182 (in which Ln and Lnxe2x80x2 are respectively a rare earth element selected from the group of Y, La, Gd and Lu; M(I) is an alkali metal selected from the group of Li, Na, k, Cs and Rb; X and Xxe2x80x2 are respectively a halogen atom selected from the group of Cl, Br and I; a and x are respectively numbers meeting the following conditions, 0.1 less than axe2x89xa610.0 and 0 less than xxe2x89xa60.2;
(12) a cerium activated rare earth halophosphate phosphor represented by the formula of LnPO4.aLnX3:xCe3+, as described in JP-A No. 61-40390 (in which Ln is a rare earth element selected from the group of Y, La, Gd and Lu; X is a halogen atom selected from the group of F, Cl, Br and I; a and x are respectively numbers meeting the following conditions, 0.1 less than axe2x89xa610.0 and 0 less than xxe2x89xa60.2;
(13) a divalent europium activated cesium rubidium halide phosphor represented by the formula of CsX:aRbXxe2x80x2:xEu2+, as described in JP-A No. 61-236888 (in which X and Xxe2x80x2 are individually a halogen atom selected from the group of Cl, Br and I; a and x are respectively numbers meeting the following conditions, 0.1 less than axe2x89xa610.0 and 0 less than xxe2x89xa60.2;
(14) a divalent europium activated complex halide phosphor represented by the formula of M(II)X2.aM(I)Xxe2x80x2:xEu2+, as described in JP-A No. 61-236890 (in which M(II) is an alkaline earth metal selected from the group of Ba, Sr and Ca; M(I) is an alkali metal selected from the group of Li, Rb and Cs; X and Xxe2x80x2 are respectively a halogen atom selected from the group of Cl, Br and I; a and x are respectively numbers meeting the following conditions, 0.1 less than axe2x89xa620.0 and 0 less than xxe2x89xa60.2.
Of the foregoing stimulable phosphors, an iodide-containing divalent europium activated alkaline earth metal fluorohalide phosphor, iodide-containing divalent europium activated alkaline earth metal halide phosphor, iodide-containing rare earth element activated rare earth oxyhalide phosphor and iodide-containing bismuth activated alkali metal halide phosphor exhibit stimulated emission having relatively high luminance.
Radiation image conversion panels using these stimulable phosphors, after storing radiation image information, release stored energy by scanning with stimulating light so that after scanning, radiation images can be again stored and the panel can be used repeatedly. In conventional radiography, a radiographic film is consumed for each photographing exposure; in the radiation image conversion method, however, the radiation image conversion panel is repeatedly used, which is advantageous in terms of natural resource conservation and economic efficiency.
It is therefore desirable to provide performance capable of withstanding for the use over a long period of time, without deteriorating radiation image quality, to the radiation image conversion panel. However, in general, stimulable phosphors used in the radiation image conversion panel are so hygroscopic that when allowed to stand in a room under usual climatic conditions, the phosphor absorbs atmospheric moisture and is deteriorated over an elapse of time. Exemplarily, when the stimulable phosphor is allowed to stand under high humidity, radiation sensitivity is lowered along with an increase in absorbed moisture content. In general, radiation latent images recorded onto the stimulable phosphor, after being exposed to radiation rays, regress over an elapse of time and the period between exposure to radiation rays and the phosphor exhibits such behavior that scanning with stimulating light requires longer time, the intensity of reproduced radiation image signal becomes less, so that moisture absorption of the stimulable phosphor accelerates the foregoing latent image regression. Accordingly, the use of a radiation image conversion panel having such a moisture-absorbing stimulable phosphor often lowers reproducibility of reproduced signals at the time of reading radiation images.
It is generally known that stimulability of stimulable phosphor particles depends on their particle sizes and the preferred average particle size is 1 to 30 xcexcm. The relationship between the average particle size and characteristics such as sensitivity, graininess and sharpness is disclosed in JP-B No. 3-79680 (hereinafter, the term, JP-B refers to a published Japanese Patent).
An attempt to control the size or form of stimulable phosphor particles in the liquid phase process is disclosed in JP-A 7-233369. In the preparation of rare earth activated alkaline earth metal fluorohalide stimulable phosphors, the conventional method is that raw materials such as an alkaline earth metal fluoride, an alkaline earth metal halide other than the fluoride and a rare earth element are mixed in a dry process or dispersed in an aqueous medium, thereafter, the mixture is calcined and ground. On the contrary, there is disclosed a liquid phase process, in which a rare earth activated alkaline earth metal fluorohalide phosphor is precipitated in an aqueous solution. The foregoing liquid phase process enables to obtain rare earth activated alkaline earth metal fluorohalide stimulable phosphor particles of small and homogeneous particle size with no deterioration in performance due to grinding.
However, enhancing sensitivity or rendering particles smaller produced problems such as deterioration due to moisture. The deterioration is initiated at the moment when the phosphor particles, after calcination, are exposed to the atmosphere and to prevent such a deterioration, storage of calcined phosphor particles under an environment screened from the atmosphere was contemplated but it was essentially difficult to conduct the whole process of preparing a phosphor plate under such an environment. To prevent the foregoing deterioration in performance of stimulable phosphor particles due to moisture absorption, there were proposed the use of a titanate-type coupling agent described in JP-B No. 2-27819, the use of silicone oil described in JP-B 5-52919, and a technique described in JP-B 11-270155, in which stimulable phosphor precursor particles were calcined in the presence of metal oxide particles and the calcined phosphor particles were covered with a metal alkoxide. However, none of these proposals led to viable solution.
Accordingly, it is an object of the invention to overcome the foregoing problems arisen with radiation image conversion panels using stimulable phosphors and to provide a preparation process of phosphor particles which are easily subjected to a moisture-proofing treatment and used for radiation image conversion panels with no deterioration in performance due to moisture absorption and usable in a viable state over a long period of time.
The above object of the present invention can be achieved by the following constitutions:
1. A preparation process of a rare earth activated alkaline earth metal fluorohalide stimulable phosphor particles represented by the following formula (1):
xe2x80x83Ba(1-X)M2xFX:yM1, zLnxe2x80x83xe2x80x83formula (1)
wherein M2 is at least one alkaline earth metal selected from the group consisting of Mg, Ca, Sr, Zn and Cd; M1 is at least an alkali metal selected from the group consisting of Li, Na, K, Rb and Cs; X is at least one halogen atom selected from the group consisting of Cl, Br and I; Ln is at least one rare earth element selected from the group consisting of Ce, Pr, Sm, Eu, Gd, Tb, Tm, Dy, Ho, Nd, Er and Yb; and x, y and z are numbers within the range of 0xe2x89xa6xxe2x89xa60.6, 0xe2x89xa6yxe2x89xa60.05 and 0 less than zxe2x89xa60.2, respectively;
the process comprising:
preparing particles of a precursor of the stimulable phosphor in a liquid phase process,
covering the precursor particles with fine particles of at least two kinds of metal oxides, and then
subjecting the precursor particles to calcination;
2. A rare earth activated alkaline earth metal fluorohalide stimulable phosphor particles represented by the foregoing formula (1), covered with fine particles of at least two kinds of metal oxides and prepared by the process described in 1. above;
3. A radiation image conversion panel comprising a support having thereon a phosphor layer containing the stimulable phosphor particles prepared by the process described in 1. above.
Further, preferred embodiments of the invention are as follows:
4. The rare earth activated alkaline earth metal fluorohalide stimulable phosphor particles described in 2. above, wherein the total amount of the metal oxide particles 0.001% to 10% by weight, based on the stimulable phosphor precursor, and the ratio of the two kinds of metal oxides is within the range of 1:2 to 1:10;
5. The preparation process described in 1. above, wherein the average size of the metal oxide particles is 2 to 50 nm;
6. The preparation process described in 1. above, wherein the total amount of the metal oxide particles 0.001% to 10% by weight, based on the stimulable phosphor precursor.